Wavelength-division multiplexing techniques are employed for better utilization of the bandwidth of optical fibers and other waveguides. In order to be able to access different wavelength channels in a ring structure, an optical signal or power that is transmitted on one channel and carries, for example, message or signaling information must be essentially removed (dropped). The channel from which the optical signal has been dropped can be occupied with a new optical signal or power (added) that, for example, can contain message or signaling information. This method of removing an optical signal from a channel and then re-occupying this optical channel is known as the add/drop method.
There are various possibilities for realizing such an add/drop method: a) the optical signals transmitted on the various wavelength channels are wavelength-division multiplexed and the add/drop method is implemented with the assistance of a suitable waveguide structure and, subsequently, the optical signals--including the optical signal re-occupying the one channel--are again multiplexed; or b) a passive optical add/drop filter, for example a Mach-Zehnder Interferometer composed of, for example, planar quartz glass waveguides, is employed.
In transmission systems in which there is no fixed channel allocation, adjustable or, respectively, tunable waveguide structures or filters must be employed. In Case a), this can be achieved by using the add/drop method with the assistance of an array of optical switches, which comprises the waveguide structure, or that is present in addition to the waveguide structure. In Case b), the method is implemented with the assistance of an adjustable or, respectively, tunable add/drop filter.
The adjustable add/drop filter is a key component for wavelength-division multiplex networks (WDM networks). This component enables a logical WDM network topology to be switched without modifying the physical topology. This property can be employed for rebooting in case of an outage at the node or of a connection, in case of a temporary increase in the capacity between two points in that a plurality of channels are allocated to this connection, for adapting the network to the requirements of the user on a software basis and/or for producing temporarily unoccupied channels between two devices with optical interfaces.
When setting or tuning a traditional optical add/drop module for locating the desired wavelength channel, all channels between the previously set channel and the new channel to be located are disturbed for a certain time. As a result, the communication is interrupted and the application of adjustable or, respectively, tunable, passive add/drop devices becomes impractical.